Stripline gain equalizer



Dec. 15, 1970 J. L. PUTZ STRIPLINE GAIN EQUALIZER 2 Sheets-Sheet 1 FiledJuly 28, 1967 FREQUENCY FREQUENCY;

- INVENTOR. mm L. PUTZ FREQUENCY Dec. 15, 1970 J. L. PUTZ STRIPLINE GAINEQUALIZER 2 Sheets-Sheet 2 Filed July 28, 1967 FIG. INVENTOR.

JOHN L. PUTZ I18 L9 2.0 2.'| FREQUENCY (GHZ) United States Patent U.S.Cl. 333-28 11 Claims ABSTRACT OF THE DISCLOSURE Absorption typestriplinegain equalizers for providing a frequency sensitive loss characteristicare realized by utilizing one or more similar stripline resonator pairshaving their major axes disposed parallel to a main transmissionstripline conductor and axially spaced relative to each other such as toprovide cancellation of electromagnetic wave energy reflections causedby the reactive perturbations of the individual resonators of each pair.Various types of lumped and distributed loss mechanisms are added to theresonators to control the resonator Qs and resonance characteristics.The coupling adjustment means between the main transmission striplineand the individual resonators may be provided in a simple manner and theindividual resonators of a given pair may be n \/4 or n A/2 resonatorswhere n is any odd integer and n is any integer and both 11 and 11 arepreferably 1. In each case the individual resonators of a given pair arepreferably axially displaced along the propagation axis of the maintransmission stripline such that their respective common ends areaxially spaced n)\/ 4 where n is any odd integer and preferably 1.

BRIEF DESCRIPTION OF THE INVENTION The requirements of users of highfrequency electron discharge devices such as traveling wave tubeamplifiers constantly become more demanding with regard to acceptabledevice operating characteristics as the users apply the devices to evermore complex and restrictive system design limits. The gain vs.frequency characteristic of traveling wave tubes is a prime example. Thetube designer can go just so far in obtaining a flat, e.g., tapered,variable, etc., gain vs. frequency characteristic over the operatingband of the tube without running into excessive cost problems if theindividual desires of each user are to be satisfied by the tube designper se. Therefore the need for a cheap, compact equalizer is readilyapparent. The equalizer teachings of the present invention also haveseparate utility in any microwave system which can benefit by means forcontrolling the signal amplitude vs. frequency characteristic.

The basic design philosophy of the present invention is as discussed inthe abstract to couple one or more stripline resonator pairs to a maintransmission stripline with the major resonator axes disposed parallelto the energy propagation axis of the main stripline. The individualresonators are provided with selective R.F. absorption means of either alumped or distributed nature to control the absorption characteristicsas desired and to provide the desired resonator Qs as well as to controlthe individ ual response shapes of a given resonator pair. Theindividual resonators of a given pair are axially displaced such thatthe respective common ends are spaced nA/4 apart, where n is any oddinteger, although preferably 1, as determined at the self-resonantfrequency of the individual resonators of a given pair. The terminologycommon ends is herein defined to mean electrically similar ends which inthe case of the half-wavelength Patented Dec. 15, 1970 "ice open endedresonators means either end and in the case of the M4 ends means eitherthe respective open ends I or the respective shorted ends. This providesoptimum cancellation of the reflected R.F. energy introduced by theindividual resonators of a pair when the individual resonators aresubstantially identical and thus a minimal V.S.W.R. for each insertedpair. If the self-resonant frequencies of the individual resonators of apair differ or if the selective R.F. absorption or resistive loss levelsof the individual resonators of a pair difler, the V.S.W.R. will bedegraded accordingly. A plurality of individual pairs can advantageouslybe disposed along a single main transmission stripline to provide anydesired loss vs. frequency characteristic while still retaining the lowV.S.W.R. of a single pair. The individual resonators themselves may beprovided with means for varying the degree of coupling to the maintransmission stripline as well as with means for varying the individualresonator self-resonant frequencies. The individual resonators are n \/4and ri k/2 types where n is any odd integer and n is any integer, andwhere both n and 11 are preferably 1, as determined at the desiredself-resonant frequency of the individual resonators. The striplineelements are preferably simple fiat types but other variations such asrods etc., are not excluded and are included in the terminologystripline conductor and may be used to advantage especially in thetunable resonators as will be set forth in more detail hereinafter inthe detailed description.

It is, therefore, an object of the present invention to provide anabsorption type stripline equalizer with an improved V.S.W.R.characteristic within the design band of the equalizer.

A feature of the present invention is the. provision of an absorptiontype equalizer incorporating a main transmission stripline with at leastone pair of stripline resonators coupled thereto with the individualstripline resonators provided with R.F. absorbing means and having theirmain axes disposed substantially parallel to the propagation axis of themain transmission stripline with the ends of the individual resonatorsof a pair axially displaced to cancel the R.F. reflections introduced bythe individual resonators of a pair.

These and other features and advantages of the present invention willbecome more apparent upon a perusal of the following specification takenin conjunction with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 depicts a fragmentary andpartially sectioned high frequency electron discharge device of thetraveling Wave amplifier type incorporating an equalizer at the R.F.input port.

FIG. 2 is an enlarged perspective view of an equalizer incorporating theteachings of the present invention.

FIG. 3 is a fragmentary perspective view of a stripline resonator havingmeans for varying the self-resonant frequency of the resonator.

FIG. 4 is an illustrative graphical protrayal of a nonsaturated gain vs.frequency characteristic for a traveling wave tube.

FIG. 5 is an illustrative graphical portrayal of loss vs. frequencycharacteristics realizable over the design band for a typical equalizerof the present invention.

FIG. 6 is an illustrative graphical portrayal of the V.S.W.R. vs.frequency characteristics for a single resonator and a properly spacedpair of matched resonators.

FIG. 7 is a perspective view of an embodiment of a \/2 striplineresonator incorporating lumped R.F. loss means and means for varying theself-resonant frequency of the resonator.

FIG. 8 is a perspective view of a variation of the embodiment depictedin FIG. 7.

FIG. 9 is a perspective view of a M 4 stripline resonator incorporatinga lumped resistor.

FIG. 10 is a perspective view of a \/2 resonator having series R.F. lossmeans.

FIG. 11 is a plan view of another equalizer embodiment incorporating theteachings of the present invention.

FIG. 12 is another plan view of a stripline equalizer incorporating theteachings of the present invention.

FIG. 13 is a loss vs. frequency plot for the equalizer depicted in FIG.12.

DETAILED DESCRIPTION OF THE INVENTION Turning to FIG. 1 there isdepicted a high frequency electron discharge device of the travelingwave type 10 including any conventional beam forming and projectingmeans 11 disposed at the upstream end portion thereof and beam collectormeans 12 disposed at the downstream end portion thereof. Disposed withina tube envelope 13 between the upstream and downstream end portions ofthe device is a slow wave interaction circuit 14, e.g. a ring-and-barcircuit supported on dielectric rods 15 in a conventional manner. A PPMfocusing means 16 or any other suitable focusing means may be employedto control the beam along the device axis. Coaxial input and output portor terminal means 17 and 18 are coupled to the respective ends of theslow wave circuit 14 for introducing R.F. energy for amplification andfor extracting the amplified energy.

An equalizer 20 is coupled to the R.F. input port 17 as shown. Theequalizer could be coupled to the output port if desired. However, thepower handling capabilities would have to be increased accordingly. Thefunction of the equalizer is to introduce variable attenuation vs.frequency within the operating band of the tube. The precise variableattenuation vs. frequency characteristic introduced by the equalizerwill depend on the tube buyer and user specifications or it could simplybe tailored by the tube builder to provide an e.g. fiat gain vs.frequency characteristic over the operating band of tube as a standardpractice.

An absorption type stripline equalizer 20 could be of the type shown inFIG. 2 which includes a main transmission stripline 21 with a pair ofthe stripline resonators 22, 23 coupled thereto with an approximately M4axial overlap as determined at the self-resonant frequency of theindividual resonators. This one-quarter electrical wavelength overlap asdiscussed previously between the individual resonators along thepropagation axis of the main stripline has been found to produce a verylow V.S.W.R. in each direction for the pair. In other words, energyreflections due to reactive discontinuities produce mutual cancellationwith the results shown in FIG. 6. Curve C is an illustrative plot ofV.S.W.R. vs. frequency for an equalizer as depicted in FIG. 2 with onlya single resonator coupled to the main line, whereas Curve D illustratesthe reduction in V.S.W.R. for a pair of resonators having the sameself-resonant frequency and a \/4 overlap where A is determined at thecommon self-resonant frequency. The reduction in R.F. energy reflectionsdue to reactive discontinuities achieved with a 50-ohm stripline designusing a pair of resonators of substantially \/2 length with a M4 overlapwas substantial. The equalizer of the above design was terminated in a50-ohm load, in both directions, and V.S.W.R. l.2:l over a 15% designband, in both directions, was measured which provides a graphicillustration of the effectiveness of the M4 overlap design in producingan essentially completely absorptive equalizer with substantially noreactive mismatch.

Turning again to FIG. 2 the stripline equalizer 20 is seen to be of acompact, simple, and rugged design enclosed in a housing 24 whichincludes a conductive wall 25 which serves as the ground plane for boththe main and coupled resonator striplines. Any conventional conductivematerials such as e.g. copper, silver-plated brass, may be used for allconductive portions of the equalizer if desired. The dielectricsubstrates 26, 27, 28 between the main stripline conductor 29 and theresonator stripline conductors 30, 31, respectively, may be of anysuitable dielectric materials such as e.g. polystyrene, Teflon, ceramic,and any suitable bonding cement such as e.g. polystyrene cement, epoxy,may be used to bond the dielectric and conductive elements of theequalizer together to form a rigid design. The remaining wall portionsof the housing 24 can be joined by any conventional means. Of course,the entire housing could be made out of an integral cup design with asingle cover member if desired. A pair of coaxial couplers 33, 34 arefastened to opposite end walls 35, 36 of the housing as shown and theircenter conductors 37, 38 are bonded to the respective ends of theconductor 29 of the main stripline 21 by any conventional metal joiningtechnique e.g. brazing. Dielectric insulation beads 40, 41 provide arigid hermetic seal between the inner and outer conductors of thecoaxial input and output couplers. Since the equalizer has essentiallyidentical response characteristics regardless of which way RF. energy isdirected through it, either port may be used as the input or output.

The degree of RF. attenuation provided within the design band of theequalizer is controlled via the introduction of R.F. attenuation or lossin the individual resonators 22, 23. For example, the resonatorconductors 30, 31 may be made of lossy conductive materials such as e.g.stainless steel, thin alloy films such as platinum, and/ or thesubstrates 27, 28 may be made of lossy dielectrics such as e.g.fiberglass, carbon loaded epoxy, and/or lossy-conductive films 42 may bedeposited directly on the substrates as shown, Suitable types oflossy-conductive films are e.g. Aquadag which can be deposited bypainting or by spraying, and metallic alloy films such as platinum ornichrome which can be deposited by evaporating techniques. The lossyfilms can be deposited near the ends of the respective resonators asshown to obtain maximum effect, since the electromagnetic E-fields arestrongest at the ends of the resonators. The lossy film conductors canextend completely around the substrates between the ground plane andabove-ground conductors or other approaches such as set forthhereinafter may be used to advantage.

If it is desired to make the individual resonators adjustable eitherfrom the standpoint of providing a fine tuning mechanism for adjustingthe individual resonances by varying the lengths of the conductors 30,31 such that the frequency at which they are M2 or self-resonant isvaried or for varying the frequency at which 11 M4 displacement occurs,the design in FIG. 3 may be used to advantage.

In brief, the tunable stripline resonator depicted in FIG. 3 includes ametal rod 44 with threaded bores at each end for receiving tuning screws45, 46 as shown. The rod 44 is affixed to the dielectric substrate 47and an elongated slot 48 is provided in the substrate for receiving apreferably dielectric clamp screw 49 which is screwed in a threaded borein the ground plane 25 (not shown). By simply turning the screws 45, 46the self-resonant frequency of the stripline resonator is varied asdesired. By loosening the screw 49 the spacing between the stripconductor (rod 44) and the main stripline can be varied as desired toadjust the individual resonator to main transmission line coupling asdesired.

FIG. 4 merely depicts an example of a response curve of gain vs.frequency for a hypothetical system, device, etc., which may be tailoredas desired through use of an equalizer incorporating the teachings ofthe present invention. For example, if the equalizer user wants tocontrol the gain to the limits between A and B over the frequency rangebetween f and f so that the shadowed region above A is lowered, hemerely tailors the loss vs. frequency characteristic of the equalizer toobtain e.g.,

curve 1 depicted in FIG. 5. If the equalizer user wants a more or lessequal overall loss introduced across a given band, curve 2 in FIG. maybe of interest. If the equalizer user wants an asymmetric loss vs.frequency characteris tic, then the equalizer can be designed to providea loss vs. frequency response such as e.g., curve 3 in FIG 5.

Curve 1 in FIG. 5 is illustrative of equal coupling between mainstripline 21 and each individual resonator with equal R.F. loss for eachresonator. Curve 2 in FIG. 5 is respresentative of stagger-tuned pluralpairs of resonators with equal coupling and loss for each individualresonator, and curve 3 is representative of staggered tuning anddifferent coupling and loss between plural pairs of resonators.Staggered tuning simply means adjusting or designing the individualstripline resonators of different pairs to slightly different resonantfrequencies by varying the self-resonant frequency as discussed above.To preserve the low V.S.W.R. response characteristics of the equalizer,the individual self-resonant frequencies, loss, and coupling of theresonators in a given pair should be made equal and variations in theabove parameters should be made between pairs. The width of a givenresponse curve for a pair is a function of the amount of loss used ineach resonator pair. The greater the loss the less sharp the responsecurve will be. The degree of attenuation or loss at midband is afunction of the coupling between the individual resonators and the mainstripline. The coupling is primarily a function of the spacing betweenthe individual resonators and the main stripline and the width of therespective above-ground conductors and can be made adjustable.Obviously, then the equalizer techniques taught herein are capable ofproviding many different variable attenuation vs. frequency responsecharacteristics.

Turning now to FIG. 7 there is depicted a variation of an individualstripline resonator which may be used to advantage. The resonator 50includes a dielectric slab 47 and adjustable rod conductor line 44 withtuning screws 45, 46 as in the FIG. 3 embodiment, disposed on groundplane 25 together with slot 48 to provide adjustable couling if desired.The R.F. loss means used in the FIG. 7 embodiment is a lumped resistor52 which is soldered or the like via terminal conductors 53, 54 whichare axially displaced along the major axis of the rod 44 as shown. Theresonator as in the previous cases is a self-resonant /2 A resonator.The Q of the resonator can be controlled by the resistance values, withsome accompanying change in the resonant frequency. The use of 500 to10,000-ohm resistors connected as shown in FIGS. 7 and 8 produces anincrease in resonator Q with increasing resistance as well as a smallreduction in resonator frequency. Values of resistance lower than acritical minimum value which can be determined by experimental empiricalapproaches will produce an opposite effect, namely an increase in Q fora decrease in resistance. Increasing the axial spacing between terminals53, 54 increases the resistor effectiveness as it will increase theE-field differential between the terminals.

In FIG. 8 another resonator 57 is depicted which is a modified versionof the FIG. 7 embodiment. The modification involves the tuning means. InFIG. 8 the selfresonant conductor 58 is a simple strip conductor e.g.copper which is bonded to a dielectric substrate 47 e.g. 6.13. T exolitecopper-clad laminate, grade 11711, manufactured by General ElectricCorporation. The selfresonant frequency of the /2 wavelength resonatorcan be varied by curling up the ends 60, 61 as shown to vary theresonator length.

In FIG. 9 a variation of a stripline resonator is depicted which is ofthe A/ 4 type as opposed to the A/ 2 types discussed previously. Theresonator 65 includes dielectric substrate 66 and strip conductors 67,25 with a lumped resistor 68 shorting the conductors together at the oneend as shown via lead terminal 69 with the other lead terminal 70soldered or the like to the conductor 67 as shown.

In FIG. 10, a N2 strip resonator 71 using a series resistor forproviding a controlled amount of loss is depicted. The line 72 issegmented at the center and a carbon paint or lossy carbon loadedceramic, etc. resistor 73 disposed therebetween as shown. This resistorserves as a part of the half-wave resonator and by varying the length orresistivity of same the effectiveness of same as a loss mechanism may beincreased.

FIG. 11 is another embodiment of an equalizer incorporating the teachingof the present invention. The equalizer 80 includes ground plane 81forming a part of a housing 82 similar to the housing shown in FIG. 2. Amain transmission stripline conductor 83 is coupled between the endterminals e.g. as shown in the FIG. 2 em- .bodiment. Two pairs ofresonators 84, 85 of the types such as discussed above are included inthe equalizer. The one pair of resonators 84 includes a pair of 2 stripconductors forming resonators 86, 87 with 4 end spacing for reasonsdiscussed previously, and the other pair of resonators includes a pairof M4 strip conductors 88, 89 again using nA/4 axial spacing betweensimilar ends, where n is an odd integer. Since the individual resonatorpairs are effectively decoupled their individual loss vs. frequencyresponses may be simply superimposed to obtain the overall resultantequalizer loss vs. frequency response characteristic. The M4 resonatorsare symmetrically oriented such as to have similar ends facing inopposite directions. This orientation will provide a bi-directional R.F.match whereas if the orientation is such that similar ends face in thesame direction, the lack of symmetry will in general result in a goodR.F. match in one direction only. For the non-symmetrical orientation,the optimum spacing along the main line may depart somewhat from theideal n7\/ 4 value previously given.

The M4 resonators of a given pair can be symmetrically disposed withtotal overlap on opposite sides of the main transmission line 83 andwith the common ends still M 4 spaced to provide a reasonable V.S.W.R.,although in general the match is inferior to that obtainable with 3M4spacing between outside ends. A possible advantage of this configurationis the attainment of a broader absorption characteristic than ispossible with a more widely disposed pair, due to the mutual couplingbetween members of the pair.

In FIG. 12 an equalizer 90 incorporating 7 individual resonators using 3similar pairs and a single resonator is depicted. The half-wave lengthpair 91 was tuned for selfresonance at 1750 me. and the loss resistorswere each 1000 ohms. The half-wave length pair 92 was tuned forself-resonance at 1820 mc. and the loss resistors were each 10,000 ohms.The )r/ 4 pair 93 was tuned for self-resonance at 1790 mc. and the lossresistors were each 5000 ohms. The overlap for the M2 pairs was )\/4 attheir respective self-resonant frequencies and the M4 pair were spacedapproximately A/ 4. A single M4 resonator 94 was made self-resonant at2120 mc. and a SOOO-ohm resistor used.

The loss in db vs. frequency characteristic for the absorption equalizerdepicted in FIG. 12 is shown in FIG. 13. The plot is self-explanatoryand indicates the high degree of flexiblity in design which can beachieved in designing stripline equalizers as taught herein. Since theequalizer package for a commercial design includes a cover, the Q,coupling, and tuning adjustments are helpful for compensating for thevariations in the self-resonant frequencies, Q and coupling produced bythe addition of the cover. If perfect symmetry with regard to V.S.W.R.is desired for a given equalizer, only matched pairs should be used andmechanical accuracy as well as control of the electrical parameters willhave to be watched carefully.

Since many changes could be made in the above construction and manyapparently widely different embodiments could be constructed withoutdeparting from the scope thereof it is intended that all mattercontained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. An absorption equalizer for microwave energy for providing apredetermined variable loss vs. frequency characteristic within thedesign band of the equalizer including, an unbroken main striplinetransmission conductor means disposed between a pair of coupling portsfor introducing and extracting electromagnetic wave energy into and outof said equalizer, at least one pair of stripline resonatorselectrically insulated from and coupled to said main striplinetransmission conductor means, the individual resonators of said pair ofstripline resonators having their major axes disposed substantiallyparallel to the major axis of said main transmission stripline conductormeans, said individual resonators of said pair of stripline resonatorseach being self-resonant at approximately the same frequency within thedesign band of the equalizer, said individual resonators of said pair ofstripline resonators each being provided with lossy R.F. attenuationmeans for absorbing R.F. energy coupled therein from said maintransmission stripline conductor means, said individual resonators ofsaid pair of stripline resonators having their common ends axiallydisplaced approximately n \/4 electrical wavelengths apart along themajor axis of the main transmission stripline conductor means where n isany odd integer and preferably 1 and where A is determined atsubstantially the self-resonant frequency of the individual resonators.

2. The absorption equalizer as defined in claim 1 wherein each of saidindividual stripline resonators include a stripline conductor which is11 M4 in length where 11 is any odd integer and preferably 1 and isdetermined at the self-resonant frequency of the resonators, each ofsaid resonators having the one end thereof electrically shorted betweenthe two conductors forming said stripline resonator.

3. The absorption equalizer as defined in claim 1 wherein each of saidindividual stripline resonators includes a stripline conductor which is11 M2 in length where 11 is any integer and preferably 1 as determinedat the self-resonant frequency of said resonators.

4. The absorption equalizer as defined in claim 1 wherein said lossyR.F. attenuation means includes axially spaced lossy conductive coatingsdeposited on the resonator surface portions.

5. The absorption equalizer as defined in claim 1 wherein said lossyR.F. attenuation means includes at least one lumped resistor shuntedacross a single stripline conductor of each of said resonators.

6. The absorption equalizer as defined in claim 1 wherein said lossyR.F. attenuation means includes at least one lumped resistor shuntedbetween a pair of conductors forming each of said stripline resonators.

7. The absorption equalizer as defined in claim 1 wherein saidresonators are provided with coupling adjustment means for varying thecoupling between the main stripline transmission conductor means and theindividual resonators.

8. The absorption equalizer as defined in claim 1 wherein saidresonators are provided with means for varying the individualself-resonant frequencies of said resonators.

9. The absorption equalizer as defined in claim 1 wherein saidabsorption equalizer includes a conductive ground plane forming a commonconductor and support base for said main transmission striplineconductor means and said individual stripline resonators, said commonground plane forming a wall portion of a housing structure for saidabsorption equalizer, said individual stripline resonators and said maintransmission stripline conductor means including dielectric substratesdisposed on said common ground plane with the frequency determiningconductor means being disposed on said dielectric substrates, saidfrequency determining conductor means being provided with means foradjusting the self-resonant frequencies of said individual resonators.

10. The absorption equalizer as defined in claim 1 wherein a pluralityof pairs of stripline resonators are coupled to said main transmissionstripline conductor means with the individual resonators of each of saidpairs of resonators having approximately the same self-resonantfrequency and wherein the common ends of the frequency determiningconductors of the individual resonators of a given pair of each of saidplurality of pairs of resonators are axially displaced approximatelyIl)\/4 along the major axis of said main transmission striplineconductor means where n is any odd integer and where is determined foreach pair of resonators at the self-resonant frequency of the individualresonators of that pair of resonators.

11. The absorption equalizer defined in claim 1 wherein said equalizerincludes resonator pairs of n )\/4 and n M2 coupled to said maintransmission stripline conductor means at axially displaced sectionsalong the major axis of said main transmission stripline conductormeans, where 11 is any odd integer and preferably 1 and n is any integerand preferably 1 and where A is determined for a given pair at a commonself-resonant frequency of the resonators of a given pair of resonators.

References Cited UNITED STATES PATENTS 2,984,802 5/1961 Dyer et al.2,820,206 1/1958 Arditi et al. 2,859,417 11/1958 Arditi. 2,937,3475/1960 Matthei et al. 3,104,362 9/1963 Matthei 333-73 3,215,958 11/1965Isaacson. 2,961,621 11/1960 Tannenbaum 33--81 HERMAN KARL SAALBACH,Primary Examiner C. BARAFF, Assistant Examiner US. Cl. X.R.

